In the classical railroad air brake system, as developed from the Westinghouse air brake, the brake air line, which passes from the locomotive and from car to car down the length of the train, provides two basic functions.
First, it is used to charge compressed air tanks in the railroad cars. The air stored in these tanks provides the energy needed to apply the brake shoes when a brake application is required. When the train is running normally, and no brake application is needed, a high pressure in the range from 70 to 110 pounds exists in the brake air line. The tanks in the cars are charged to the same pressure as the air in the brake air line.
Second, when a brake application is required, air is exhausted from the brake air line, causing the pressure in the brake air line to be reduced. In the cars of the train, this reduction of pressure is used as a signal to apply the brakes. In this event, valving in the cars utilizes the compressed air in the tanks to apply pressure to the brake shoes so that the brakes are applied.
After a train has been stopped by an application of the air brakes, the air pressure in the tanks on the cars of the train is depleted. In order for the train to operate safely, the engineer must wait until the tanks are recharged before he puts the train in motion.
In order for the engineer to know when the tanks are charged, a flow measuring system is used to indicate the flow rate of air from a compressor in the locomotive to a valve which supplies air to the brake pipe in the locomotive. When this flow stops, the engineer knows that the tanks in the cars are fully charged, and that it is safe to proceed.
A flowblock is installed in the line from the main reservoir to the valve which supplies air to the brake pipe. In the flowblock, the air flow is passed through an orifice, and two transducers are used to obtain pressure information which is supplied to a computer which calculates the air flow.
Generally, an upstream transducer measures pressure at a pressure tap in the line upstream of the orifice. In addition, a downstream transducer responsive to pressure obtained from a tap in the orifice is used in conjunction with the upstream transducer in order to obtain sufficient data to calculate the air flow. The downstream transducer may be either one of two different types, depending on the accuracy which is required. The first of the two types is a simple pressure transducer which measures the pressure in the orifice. The difference between the pressure upstream of the orifice, and the pressure in the orifice is used as a basis for calculating the flow rate of the air. A person skilled in the art will recognize that Bernoulli's equation can be used to calculate a value for the flow rate of the air. The second of the two types is a differential transducer which measures the pressure difference between the upstream pressure tap, and the pressure tap at the orifice.
A flow measurement system in which the downstream transducer is a differential transducer is more accurate than a flow measurement system in which the downstream transducer is a simple pressure transducer because a system using a differential transducer does not require subtraction of the numerical values of two independent physical measurements. It is desirable to avoid subtraction because subtraction generally degrades the accuracy of the final result because the percentage error in the difference of two quantities having the same sign is greater than the individual percentage errors in either of the two quantities.
Regardless of which type of system is used, two transducer signals are supplied to analog-to-digital converters in the computer. For either type of system, one or more programs in the computer use the digital outputs of the converters to calculate the flow rate.
In the prior art systems, there is no automatic method for the computer to determine which of the two types of transducer is employed as the downstream transducer. This information must be determined in order for the computer to properly process the transducer information to obtain flow rate.
To provide additional information on railroad airbrake systems, as background for the present patent, the teachings of the following United States patents are incorporated herein by reference thereto; in addition to the copending application cited above.
U.S. Pat. No. 4,904,027 by Skantar and Sanders: DIGITAL AIR BRAKE CONTROL SYSTEM.
U.S. Pat. No. 5,192,118 by Balukin, Newingham and Jerina: ELECTRO-PNEUMATIC LOCOMOTIVE BRAKE CONTROL SYSTEM.
U.S. Pat. No. 5,222,788 by Dimsa and Jenets: MICROPROCESSOR BASED ELECTRO-PNEUMATIC LOCOMOTIVE BRAKE CONTROL SYSTEM HAVING BRAKE ASSURANCE CIRCUIT.
Each of these patents is assigned to the assignee of the present invention.